The Automatic Registration 2 module is used to undertake an initial
and coarse co-registration of the individual scans into the same
coordinate system. The data used are: * the RTK-corrected observations
collected from the onboard GNSS receiver * inertial measurements
collected from the onboard IMU * planes derived from voxels generated
across the point clouds themselves. The GNSS/IMU data provide an initial
solution on the 3D position of the scanner, itself further refined using
plane-to-plane matching.
1. Unregister any prior registration
R-click on scans → Unregister
No ScanPos should have a
green circle next to it anymore
2. Delete bad scans
Bad scans should be
deleted. Every ScanPos should have only 1 scan in it. Bad scans can be
caused by wind or scanner movement. The general rule is that the scan
with the oldest timestamp is the correct. Check the field sheets if
unsure.
No ScanPos should have a green circle next to it anymore
3. Filter scans
Open the One-Touch Processing
Wizard
Check the box for Filter Scans and use the
following settings: * Keep all echoes per laser shot * Delete all point
with a reflectance below -20 dB * Delete all point with a reflectance
above 5 dB * Delete all point with a deviation above 20 (the deviation
filter can vary, but is generally in the 12 to 20 range). Make sure to
document this value.
4. Automatic Registration
Registration →
Automatic Registration 2
* Project scenario: Custom settings *
Voxel Size: 0.1 m * Voxel Count: 1024 * Manual assistance: Never *
Scanner transportation: Carried manually
5. Fix the Automatic Registration After the
Automated Registration, you will notice that a few scans are not
registered (i.e. they will not have the green circle next to the
ScanPos). There are 2 general approaches to fix them. First fix the
upright ones, then fix the tilted ones.
5.1 Anchor
points to (try to) fix broken unregistered scans
The
Automated registration 2 tries to register scans in numerical order. In
the case this was not the reality (e.g. went back to a previous scan a
number of ScanPos ago) or AR2 did not work on an upright scan: e.g. in
this example, ScanPos19 was not registered initially, but ScanPos021
was.
This was fixed by setting an anchor point from ScanPos19 to
ScanPos21: R-click → set anchor point → pick anchor
Re-run
AR2. The order of automatic registration has now changed from ScanPos018
→ ScanPos021 → ScanPos019 You can play a bit around with this. The idea
is that the 2 positions that you anchor are close in space.
5.2 Tilted
scans through SOP manipulation
Fix tilted scans (they are
harder to automatically register as they can be challenging for the
IMU).
We fix
those first using the following steps. Here we demonstrate it for
up-tilt pair ScanPos59 (up, registered) and ScanPos60 (tilt, not
registered):
1. Open the SOP of the registered scan and write down the
translation coordinates (the 4th column, one decimal precision is
ok)
2.
Open the SOP of the unregistered scan, and change the 3 translation
coordinates to the ones from the registered scan. You will see that the
scans already quite good now.
3. Extract plane patches for both scans
now. Select them, right click → Extract plane patches
4. Use Multi Station
Adjustment (registration → Multi Station Adjustment → Start adjustment)
on the 2 scans. Only have those 2 scans activated, and lock the position
and orientation of the up scan. We recommend to use the settings in the
image below and start with a search radius of 1m. Click calculate to
execute the first iteration of MSA
5. After the first iteration the
registration should have improved. Now change the search radius to
approximately 2 x error (stdDev). In our example, this is roughly
0.4